Installing Recaro seats. Bolt threads issue

[centerpunch]

A 7/16 bolt would not “fit perfectly” in an M10 nut, it would really take some effort to cram that in. Not sure you could do it at all.

My guess is that a 3/8” bolt was misfiled in the 7/16” bin in the hardware store and that’s what you used. (You also say you used a 3/8 washer.)

I have noticed that a 3/8-16 bolt will fit almost perfectly in a M10-1.5 nut, a little loose, but easy to ignore if you’re not paying attention. So might also kinda fit M10-1.25. I have actually written warnings on my thread measuring display (search “thread checker” on Amazon) to remind myself not to do that.

 

[centerpunch]

Oh, and btw, fine threads are stronger than coarse threads.

 

[Wild Turkey]

Oh, and btw, fine threads are stronger than coarse threads.

Good point - thanks for the correction. I did not state it well. I had assumed the larger diameter bolt would be stronger regardless, but with different thread pitches and potentially different grades of bolts, it's impossible to know, and like-for-like fine threads are stronger.

But your thought that it might be a miss-binned 3/8" bolt in the 7/16" bin is brilliant, could make more sense, and could make most of our above discussion inapplicable, with the exception that it's time to get a mechanic involved.

 

[centerpunch]

I’m a safety-conscious retired mechanical engineer, and I might be off-base, but I wouldn’t worry too much about one tight-but-maybe-not-quite-right rear seat bolt.

The front bolts are very important if you get hit from the rear- holding the seat in place so they don’t tear loose, preventing the seat (and you) from rocking back unsecured.

In a front impact, the seatbelts do the bulk of the work in restraining you, with the seat really just along for the ride.

In a side impact, seems like the weight of the occupant (secured by seatbelt) is much more significant than the weight of the seat.

Bottom line, if that mismatched bolt tightened up ok, I’d just leave it alone. That seat secured by three and a half bolts will be fine.

(In my opinion, that is.)